nfp: bpf: fix code-gen bug on BPF_ALU | BPF_XOR | BPF_K

The intended optimization should be A ^ 0 = A, not A ^ -1 = A.

Fixes: cd7df56e

 ("nfp: add BPF to NFP code translator")
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>